The present invention relates to a thermoplastic composite material consisting of reinforcing fibers and a thermoplastic resin and having an improved toughness, and a method of producing such a composite material.
Various composite materials consisting of reinforcing fibers such as carbon fibers and matrix phases of thermoplastic resins are used in various fields recently. Since those containing thermoplastic resins as matrix resins are easily formed, they are used widely for various home electric appliances, machine parts, automobile parts, structural materials, etc. Particularly, since poly(ether ether ketone) (hereinafter referred to as "PEEK"), one of the thermoplastic resins, shows excellent mechanical strength, elongation, damage tolerance, etc., composite materials containing the PEEK as a matrix resin are expected to be used for various machine parts including automobile parts, structural materials, etc.
With respect to the reinforcing fiber layer, there is provided a sheet consisting of reinforcing fibers such as carbon fibers and fibers such as PEEK fibers. When this sheet is used for composite materials, it is laminated in a plural number, and compressed while heating so that the PEEK is melted into an integral phase.
The composite materials constituted by reinforcing fibers and PEEK matrices are somewhat poor in impact resistance (toughness), in spite of having good mechanical strength. In order to improve the toughness of the composite materials consisting of the reinforcing fibers and the PEEK, one means is to use PEEK having a high molecular weight. However, since the high-molecular weight PEEK shows a high melt viscosity, molten PEEK does not sufficiently enter into the reinforcing fibers. If a low-molecular weight PEEK is used, the resulting composite material fails to show sufficiently improved toughness as compared with the use of the high-molecular weight PEEK. Nevertheless, in conventional reinforcing fiber/PEEK woven sheets, low-molecular weight PEEK fibers are used, to ensure that the reinforcing fiber sheets are well impregnated with the PEEK. Accordingly, composite materials having excellent mechanical properties such as toughness are desired.